1. Field of the Invention
This invention relates to an optical time domain reflectometer with an automatic level adjusting function and, more particularly, to an optical time domain reflectometer which detects backscattered light and Fresnel reflection light, generated by an optical pulse sent to a target optical fiber and returning from the fiber, to thereby measure the loss, and find a crack or cut point, in the target optical fiber.
2. Description of the Related Art
A conventional optical time domain reflectometer tests a target optical fiber by sending an optical pulse to the target optical fiber and detecting Fresnel reflection light and backscattered light returning from the fiber.
FIG. 1 schematically illustrates the arrangement of this optical time domain reflectometer. In the figure, reference numeral 10 is a pulse generator for generating a pulse signal, 20 is an optical output section for generating an optical pulse according to the pulse signal, and 30 is a light-path switching section for sending the optical pulse to target optical fiber 40 and extract Fresnel reflection light and backscattered light returning toward the pulse-launching end of the fiber from inside the fiber; a directional coupler is used as this component 30 in this example. Reference numeral 51 is a photoelectric converter for converting Fresnel reflection light and backscattered light into an electric signal, 52 is a first amplifier for amplifying the electric signal, 53 is a variable resistance attenuator, 54 is a second amplifier for amplifying an electric signal from this attenuator 53, 60 is an A/D converter for converting the electric signal into a digital signal, 90 is a display section, and 100 is a data processor.
FIG. 2 gives a typical characteristic of a target optical fiber which is acquired by the above arrangement, and it shows level (dB) on the vertical scale and distance (km) on the horizontal scale with F.sub.0 indicating Fresnel reflection occurred at the pulse-launching end of target optical fiber 40 and F.sub.1 indicating Fresnel reflection occurred at the other end. Characteristic A is of the backscattered light and its inclination indicates the light transmission characteristic of target optical fiber 40. S1 to S4 indicate spliced points of target optical fiber 40 by fusion splicing.
In the above arrangement, variable resistance attenuator 53 attenuates an electric signal which is attained by photoelectric conversion of reflection light that is determined by the type of target optical fiber 40, observation range, and the pulse width, wavelength and output level of an optical pulse from optical output section 20, to thereby prevent saturation of the electric signal in second amplifier 54. With the conventional optical time domain reflectometer, while visually confirming on the screen of display section 90 that the electric signal is not saturated in second amplifier 54, an operator manually changes and adjusts the amount of attenuation made by variable resistance attenuator 53 in such a manner that the upper left part of the characteristic curve A of target optical fiber 40 does not exceed, for example, a reference value C in FIG. 2.
This will be discussed in more detail below. The measured waveform or characteristic of a target optical fiber has its level gradually decreasing as the measured distance gets farther and locally includes high-level of pulse or the like fresnel reflection. The operator may also use a function to partially enlarge or reduce the observation range as one of variable parameters. Accordingly, the operator should operate the variable resistance attenuator 53 in view of which parameter is to be changed.
With the above optical time domain reflectometer, in converting the Fresnel reflection light and backscattered light from target optical fiber 40 into an electric signal and supplying the signal to A/D converter 60 after amplifying it, if the operator erroneously sets larger the attenuation amount for variable resistance attenuator 53, a reduction in level of the electric signal would impair the S/N ratio. To improve the S/N ratio, therefore, data processor 100 requires a significant amount of time in executing an averaging process.
On the other hand, if the attenuation amount for variable resistance attenuator 53 is set small, the electric signal is saturated in second amplifier 54 so that the Fresnel reflection light and backscattered light from target optical fiber 40 cannot be observed with fidelity.
Further, if the operator is a novice, he should have difficulty in discriminating whether or not a variable attenuation amount set by him is the proper one and should unnecessarily take time in setting the proper attenuation amount, resulting in inefficient measurement.
With the attenuation amount set different from the proper value, even when sampling is done to the same target optical fiber using the same parameter excluding this attenuation amount, there occurs a difference in S/N ratio of sampled signal levels and a variation in measurement results. Therefore, if the sampling is performed with the properly set attenuation amount and a high S/N ratio of the signal levels, the result of the sampling would have a high accuracy, whereas if the sampling is done with an improperly set attenuation amount and a low S/N ratio, it would result in a low accuracy, thus impairing the reproducibility of the sampling result. In manually setting the attenuation amount, because of a difference in operators' ability, the accuracy of the sampling results may vary or the reproducibility of the sampled results may be impaired depending on the operator, thus resulting in a low reliability.
FIG. 3 is a block diagram illustrating another arrangement of the optical time domain reflectometer which has been actually used.
This optical time domain reflectometer comprises a timing generator 1, a light emitting section 2, a directional coupler 3, a light receiving section 4, an amplifying section 5, an A/D converter 6, an accumulator 7, a display section 8 and a data processor 9. Based on a trigger signal outputted from timing generator 1, light emitting section 2 sends an optical pulse to target optical fiber 10. This pulse generates backscattered light and Fresnel reflection light in target optical fiber 10, which are in turn received by the optical time domain reflectometer. The received backscattered light and Fresnel reflection light are amplified by amplifying section 5 and are then subjected to A/D conversion in A/D converter 6. The output of A/D converter 6 is accumulated in accumulator 7 for each sampling point. Further, each accumulated data is subjected to logarithm conversion in data processor 9 and is then displayed on display section 8, thereby carrying out various characteristic measurements such as measuring of the loss of target optical fiber 10 and finding any cut or cracked section thereof.
With the use of the above conventional optical time domain reflectometer, the amplification of the backscattered light and Fresnel reflection light received by light receiving section 4 prior to their A/D conversion in A/D converter 6, is carried out by a combination of first and second amplifiers 5a and 5b with a fixed gain and a manually variable attenuator 5c.
With the above optical time domain reflectometer, however, if the operator sets the attenuation amount for attenuator too large at the time the received backscattered light and Fresnel reflection light are amplified and transferred to A/D converter 6, the signal decreases, resulting in a low S/N ratio. Data processor 9 therefore requires extra averaging time in order to improve the S/N ratio. If the attenuation amount for attenuator 5c is set too small, on the other hand, the signal is saturated in amplifier 5b, so that the reflection light from target optical fiber 10 will not be observed. Particularly, if the operator is a novice, he should have difficulty in discriminating whether or not a variable attenuation amount set by him is the proper one and should need much time in setting the proper attenuation amount, resulting in inefficient measurement. Further, because of a difference in operators' ability, there would be a variation in the attenuation amount set for attenuator 5c depending on the operator, thus resulting in a low reliability of the sampled values.
Since the operator needs to view the screen of display section 90 to determine the proper attenuation amount, it is not always possible to set the attenuation amount to the proper value under the unattended control of an external controller.